On gradually increasing the intensity of stimulus, which may be done, as already stated, by increasing the amplitude of vibration, it will be found that, beginning with feeble stimulation, this increase is at first slight, then more pronounced, and lastly shows a tendency to approach a limit. In all this we have a perfect parallel to corresponding phenomena in animal and vegetable response. We saw that the proper investigation of this subject was much complicated, in the case of animal and vegetable tissues, by the appearance of fatigue. The comparatively indefatigable nature of tin causes it to offer great advantages in the pursuit of this inquiry. I give below two series of records made with tin. The first record, fig. 83, is for increasing amplitudes from 5 deg. to 40 deg. by steps of 5 deg.. The stimuli are imparted at intervals of one minute. It will be noticed that whereas the recovery is complete in one minute when the stimulus is moderate, it is not quite complete when the stimulus is stronger. The recovery from the effect of stronger stimulus is more prolonged. Owing to want of complete recovery, the base line is tilted slightly upward. This slight displacement of the zero line does not materially affect the result, provided the shifting is slight. [Illustration: FIG. 83.--RECORDS OF RESPONSES IN TIN WITH INCREASING STIMULI, AMPLITUDES OF VIBRATION FROM 5 deg. TO 40 deg. The vertical line to the right represents .1 volt.] TABLE SHOWING THE INCREASING ELECTRIC RESPONSE DUE TO INCREASING AMPLITUDE OF VIBRATION +---------------------+----------------+ | Vibration amplitude | E.M. variation | +---------------------+----------------+ | 5 deg. | .024 volt | | 10 deg. | .057 " | | 20 deg. | .111 " | | 25 deg. | .143 " | | 30 deg. | .170 " | | 35 deg. | .187 " | | 40 deg. | .204 " | +---------------------+----------------+ The next figure (fig. 84) gives record of responses through a wider range. For accurate quantitative measurements it is preferable to wait till the recovery is complete. We may accomplish this within the limited space of the recording photographic plate by making the record for one minute; during the rest of recovery, the clockwork moving the plate is stopped and the galvanometer spot of light is cut off. Thus the next record starts from a point of completed recovery, which will be noticed as a bright spot at the beginning of each curve. With stimulation of high intensity, a tendency will be noticed for the responses to approach a limit. [Illustration: FIG. 84.--A SECOND SET OF RECORDS WITH A DIFFERENT SPECIMEN OF TIN The amplitudes of vibration are increased by steps of 10 deg., from 20 deg. to 160 deg.. (The deflections are reduced by interposing a high external resistance.)]